1. Field of the Invention
Embodiments of the present invention relate to a method of fabricating a semiconductor package, and a semiconductor package formed thereby.
2. Description of the Related Art
As the size of electronic devices continue to decrease, the associated semiconductor packages that operate them are being designed with smaller form factors, lower power requirements and higher functionality. Currently, sub-micron features in semiconductor fabrication are placing higher demands on package technology including higher lead counts, reduced lead pitch, minimum footprint area and significant overall volume reduction.
One branch of semiconductor packaging involves the use of a leadframe, which is a thin layer of metal on which one or more semiconductor die may be mounted. The leadframe includes electrical leads for communicating electrical signals from the one or more semiconductors to a printed circuit board or other external electrical devices. Common leadframe-based packages include plastic small outlined packages (PSOP), thin small outlined packages (TSOP), and shrink small outline packages (SSOP). Components in a conventional leadframe package are shown in FIGS. 1 and 2. The illustrated components may be used for example in a TSOP package, which come standard in 32-lead, 40-lead, 48-lead and 56-lead packages (fewer leads are shown in the figures for clarity).
FIG. 1 shows a leadframe 20 before attachment of a semiconductor die 22. A typical leadframe 20 may include a number of leads 24 having first ends 24a for attaching to semiconductor die 22, and a second end (not shown) for affixing to a printed circuit board or other electrical component. Leadframe 20 may further include a die attach pad 26 for structurally supporting semiconductor die 22 on leadframe 20. While die attach pad 26 may provide a path to ground, it conventionally does not carry signals to or from the semiconductor die 22. In certain leadframe configurations, it is known to omit die attach pad 26 and instead attach the semiconductor die directly to the leadframe leads in a so-called chip on lead (COL) configuration.
Semiconductor leads 24 may be mounted to die attach pad 26 as shown in FIG. 2 using a die attach compound. Semiconductor die 22 is conventionally formed with a plurality of die bond pads 28 on first and second opposed edges on the top side of the semiconductor die. Once the semiconductor die is mounted to the leadframe, a wire bond process is performed whereby bond pads 28 are electrically coupled to respective electrical leads 24 using a delicate wire 30. The assignment of a bond pad 28 to a particular electrical lead 24 is defined by industry standard specification. FIG. 2 shows less than all of the bond pads 28 being wired to leads 24 for clarity, but each bond pad may be wired to its respective electrical in conventional designs. It is also known to have less than all of the bond pads wired to an electrical as shown in FIG. 2.
FIG. 3 shows a cross-sectional side view of leadframe 20 and semiconductor die 22 after the wire bond process. Once wire bonding is completed, a molding process performed to encase the components in a molding compound 34 and form the finished package. It is known to recess or “down-set” the semiconductor die within the leadframe, as shown in FIG. 3, in order to balance the semiconductor die against the forces of the molding compound as it flows around the die and leadframe. It is important that the semiconductor die be balanced during molding process as an imbalance can cause excessive movement of the semiconductor die under the force of the molding compound as it flows. Such movement can break or short one or more of the wire bonds 28, resulting in damage or complete failure of the semiconductor package. As there may be fifty or more wire bonds in a package, this can become a significant problem if the semiconductor die is not properly balanced during the molding process.
It is also know during the molding process in a down-set packaging configuration that a higher concentration of molding compound flows over the top of the semiconductor die in the molding process. This results in a downward force on top of the semiconductor die. Without the die attach pad 26 or other proper structural support, the die and leadframe may get forced downward until one or more of the electrical leads attaching to the die are exposed to the external environment at the bottom of the package. This again may result in damage or failure of the package.
As shown in FIGS. 2 and 3 it is typical to have bond pads 28 on first and second opposite sides of the semiconductor die for electrical coupling with their respective leads. According to industry specification and ease of design, bond pads along the first edge of the semiconductor die connect to respective pins adjacent to first edge, and bond pads along the second edge of the semiconductor die connect to respective pins adjacent the second edge.
In an effort to reduce semiconductor die form factor, it is now known to provide bond pads on a semiconductor die along only one edge of the die as shown in FIG. 4. A leadframe operating with such die may include a plurality of elongated electrical leads, extending from a first side of the leadframe, beneath the die, and terminating at a second side of the leadframe adjacent to the bond pads along the single edge of the die. However, use of such a leadframe in a COL configuration may increase parasitic capacitance and/or inductance between the elongated leads and the semiconductor die on the elongated leads. High levels of parasitic capacitance and/or inductance can adversely affect the operation of the semiconductor package.